$xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1920 x 1200, maximum 1920 x 1200
default connected 1920x1200+0+0 0mm x 0mm
   1920x1080       0.0
   1600x1200       0.0
   1280x1024       0.0
   1024x768        0.0
   800x600         0.0
   640x480         0.0
   1920x1200       0.0*

$xrandr -s 1920x1080

# screen is now cropped